In recent years, power generation technology using various types of natural energy have been developed. For example, huge ocean currents such as the Japan Current (Kuroshio) are energy resources, and ocean current power generation devices have been developed that generate power using this ocean current energy. With such ocean current power generation devices, power is generated as a result of rotary blades being rotated by the ocean current energy and a power generator connected to the shaft of the rotary blades being rotated.
One system of such an ocean current power generation device is an underwater floating type ocean current power generation device (submerged floating type ocean current power generation device), an example of which is described in Patent Document 1. This underwater floating type ocean current power generation device has a configuration in which an ocean current power generation device, configured as a floating body having a degree of buoyancy, is connected to a mooring line extending from the sea floor and, as a result, generates power by the ocean current power generation device being made to float underwater within a range allowed by the mooring line. With this underwater floating type ocean current power generation device, there is no need for huge support pillars such as those used in wind power generation devices installed on land, and a simple configuration can be achieved.
FIG. 14 is a perspective view illustrating the underwater floating type ocean current power generation device described in Patent Document 1. As illustrated in FIG. 14, this underwater floating type ocean current power generation device is configured as a floating body 1 and, a twin drum model is utilized to stabilize the posture of the floating body 1. Specifically, one floating body 1 is constituted from two ocean current power generation device main bodies 2, 2 and a structure 3 connecting these main bodies 2, 2. Each ocean current power generation device main body 2 is provided with a power generator (not illustrated) within a nacelle 4 (also called a “pod”), and a rotating shaft of a rotating blade 5 is connected to the rotor of this power generator.
The nacelle 4 is joined to each left and right end of the structure 3. A front end of a mooring line 6 is attached to a center in the horizontal direction of the structure 3, and a base end of the mooring line 6 is moored to the ocean floor. The floating body 1 floats underwater in a horizontal direction and a vertical direction within the range restrained by the mooring line 6. The buoyancy of the floating body 1 is balanced on the left and right, and the structure 3 of the floating body 1 is formed in an airfoil shape that faces the ocean current direction. As such, the floating body 1 changes direction depending on the ocean current so that the front face of the floating body 1 (the front face of the rotary blades 5) directly faces the ocean current direction. Thus, power is generated.
Additionally, this ocean current power generation device uses a downwind system in which the rotary blades 5 are disposed behind (on the downstream side of the ocean current) the nacelle 4. By disposing the rotary blades 5 farther to the downstream side than the nacelle 4 in this manner, it is easier to orient the front face of the floating body 1 (the front face of the rotary blades 5) in the ocean current direction. With underwater floating type ocean current power generation devices, it is difficult to implement a drive device for actively performing directional control of the floating body 1 (control to adjust the rotor rotating shaft in the direction of the ocean current). As such, in addition to utilizing a downwind system, a configuration is used in which shapes of the nacelle 4 and structure 3 of the floating body 1 are formed so as to be oriented in the ocean current direction, thus passively orienting the front face direction of the floating body 1 in the ocean current direction.
Additionally, the floating body 1 (the ocean current power generation device) moored by the mooring line 6 floats underwater but, at this time, the floating body 1 assumes a position where the acting force of the ocean current applied to the floating body 1, the buoyancy occurring in the floating body 1, and the tension of the mooring line 6 are balanced. In other words, buoyancy acts vertically upward on the floating body 1, the acting force of the ocean current acts in the ocean current direction (the horizontal direction) on the floating body 1, and the tension of the mooring line acts to oppose the buoyancy and the force of the ocean current. Accordingly, if the acting force of the ocean current is weak (that is, if the current velocity of the ocean current is low), the floating body 1 will rise to a comparatively shallow depth underwater, and if the force of the ocean current is strong (that is, if the current velocity of the ocean current is high) the floating body 1 will sink to a depth comparatively deep underwater.
On the other hand, in a current velocity distribution in the depth direction of the ocean current near the ocean floor, the current velocity is lower the closer the ocean current is to the ocean floor and higher the farther the ocean current is away from the ocean floor. As such, if the current velocity of the ocean current is strong, the floating body 1 will sink underwater and regain balance at a depth where the current velocity has suitably weakened in the current velocity distribution in the depth direction. Moreover, if the current velocity of the ocean current is weak, the floating body 1 will rise underwater and regain balance at a depth where the current velocity has suitably strengthened in the current velocity distribution in the depth direction.
Patent Document 1 focuses on the rising and sinking of the ocean current power generation device depending on the current velocity as described above, and recites that active depth control is made unnecessary due to the application of control whereby the position in the water depth direction of the ocean current power generation device is passively automatically adjusted (also referred to as “Passive Depth Control” or “PDC”).